Remove all code related to Bintray. It no longer works so there's no
point keeping it around.
Some of this could arguably be deprecated/disabled first/instead but:
I'm not sure I see the sense in keeping stuff around that's known to be
broken.
- make the description more generic/correct
- use "internet archive" over "archive"
- move some logic to a new `GitHubReleases` class (for consistency)
- remove some obvious comments
- extract out and move some constants
